The Vif protein is essential to HIV-1 infectivity. Viruses defective in vif are poorly infectious as cell-free virions. The importance of cell- free virus was illustrated early on by the spread of AIDS in the hemophiliac population, and is still probably a key to the high rate of contamination from infected mother to fetus. Moreover, recent studies have revealed unexpectedly high levels of plasma viremia at various stages of HIV-1-induced disease, further emphasizing the role of cell-free virus in propagating the AIDS epidemic. The vif gene is found in all primate lentiviruses, as well as in other lentiviruses like FIV, BIV and visna virus, but not in classical retroviruses. This raises the possibility that Vif participates in determining the particular behavior of lentiviruses in the host, that is, their ability to establish slowly progressive infections, despite host defences, and to induce chronic degenerative diseases. This application proposes to define the role of HIV-1 Vif in determining virus infectivity. Preliminary studies presented here suggest that the function of Vif is to permit gp160-mediated viral entry. A priority will be to confirm this result, which has important implications for orienting future experiments. To analyze the role of Vif in HIV-1 replication, a combination of analytical and functional studies will be undertaken. Biochemicals properties of the Vif protein will be determined. Specifically, the intracellular localization of Vif, its potential presence in virions, as well as its possible actions on the viral envelope glycoproteins or other factors will be defined. Functional tests will search for viral or cellular factors that alter the need for Vif. Others will look at the interchangeability between Vif proteins from various viruses, to define whether Vif acts through a cellular or a virus-specific target. In addition, the mechanisms of action of Vif will be examined by comparing wtVif and deltaVif viruses through the early steps of the infectious cycle: CD4 binding, gp120/gp41 dissociation, virus internalization, uncoating, reverse transcription. Finally, potential effects of Vif on HIV-infected cells will be investigated, in the context of vif-expressing cell lines. Once the role of Vif will have been defined, rapid assays will be used to delineate Vif functional domains. Agonists of Vif will also be searched, based on analogies with proteins known to accomplish similar functions. We strongly feel that the proposed experiments will reveal crucial information regarding the way HIV is transmitted, and induces disease in patients. Most importantly, these studies might uncover new ways of blocking HIV-1 infectivity, and therefore of controlling the spread of AIDS.